An Achilles' heel is one weak spot in someone strong. Achilles was invincible except his heel.
In later tellings, Achilles' mother dipped him in the river Styx to make him unbeatable, holding him by the heel, so that spot stayed weak. He died at Troy when an arrow hit it. Your Achilles tendon is named for him.
A great quarterback with no offensive line: that line is his Achilles' heel.
Ask: 'What is our Achilles' heel?' It finds the one weak point in a strong plan.
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